[3][4] He received his early education at Siglap Secondary School, and obtained a Senior Cambridge in 1964.
[8] Upon returning to Singapore in 1972, Koh joined the Ministry of Health (MOH) as a biomedical engineer.
[15] On 10 January 1991, Koh was appointed as commissioner of the Inland Revenue Authority of Singapore (IRAS), replacing Hsu Tse Kwang.
[18] In late 1992 and early 1993, to help businesses understand and file tax returns easily, Koh launched a new publication to help clarify tax laws, and announced new measures such as a one-stop service branch and computer-assisted appraisal system to improve efficiency.
[19][20] On 1 June 1996, Koh was appointed as the second permanent secretary for MOH, assisting Kwa Soon Bee.
[22][23] Shortly after, Koh announced that salary and promotion criteria for senior doctors will be reviewed to match with the private sector.
[24][25] On 1 April 1997, Koh was given an additional portfolio of second permanent secretary for the Ministry of Finance, assisting Ngiam Tong Dow.
[31][32] In August 1998, Koh officiated the launch of the MAS Electronic Payment System, facilitating interbank transfers.
[40] On 21 October 2001, Tharman resigned from MAS to contest in the 2001 general election as a candidate for the People's Action Party, and Koh assumed his position.
[42] On 31 May 2005, Koh retired from the civil service and stepped down as managing director of MAS, with Heng Swee Keat as his replacement.
The board will spare no effort and resources to ensure that SMRT earns back the confidence of the commuters and public.